Letter to the Editor

Discovery of a hot post-AGB star in Galactic globular cluster E3

¹ Astronomy and Astrophysics Division, Physical Research Laboratory, Navrangpura, Ahmedabad, 380009 Gujarat, India
² Department of Physics and Astronomy, National Institute of Technology, Rourkela, 769008 Odisha, India
e-mail: ranjankmr488@gmail.come-mail: acp.phy@gmail.com
³ Nicolaus Copernicus Astronomical Center, Polish Academy of Sciences, ul. Rabiańska 8, 87-100 Toruń, Poland
⁴ Sternberg Astronomical Institute of the Lomonosov Moscow State University, University av. 13, 119234 Moscow, Russia
⁵ Xinjiang Astronomical Observatory, Chinese Academy of Sciences 150 Science 1-Street, Urumqi, Xinjiang 830011, PR China
⁶ Nicolaus Copernicus Astronomical Centre, Polish Academy of Sciences, ul. Bartycka 18, 00-716 Warsaw, Poland
⁷ Department of Astronomy and Astrophysics, Tata Institute of Fundamental Research, Mumbai 400005, India

Received: 28 February 2024
Accepted: 20 April 2024

Abstract

We report a new hot post-asymptotic giant branch (PAGB) star in the Galactic globular cluster (GC) E3, which is one of the first of the identified PAGB stars in a GC to show a binary signature. The star stands out as the brightest source in E3 in the Astrosat/UVIT images. We confirmed its membership with the cluster E3 using Gaia DR3 kinematics and parallax measurements. We supplemented the photometric observations with radial velocities (RVs) from high-resolution spectroscopic observations at two epochs and with ground- and space-based photometric observations from 0.13 μm to 22 μm. We find that the RVs vary over ∼6 km s⁻¹ between the two epochs. This is an indication of the star being in a binary orbit. A simulation of possible binary systems with the observed RVs suggests a binary period of either 39.12 days or 17.83 days with mass ratio q ≥ 1.0. The [Fe/H] derived using the high-resolution spectra is ∼−0.7 dex, which closely matches the cluster metallicity. The spectroscopic and photometric measurements suggest T_eff and logg of the star as 17 500 ± 1000 K and 2.37 ± 0.20 dex, respectively. Various PAGB evolutionary tracks on the Hertzsprung–Russell (H–R) diagram suggest a current mass of the star in the range 0.51–0.55 M_⊙. The star is enriched with C and O abundances, showing similar CNO abundances compared to the other PAGB stars in GCs with the evidence of the third dredge-up on the AGB phase.